During dialysis treatment, arterial and venous access needles are typically inserted into a patient such that blood can be drawn from the patient through the arterial access needle, flown through a dialyzer to filter the blood, and then returned to the patient through the venous access needle. In some cases, the venous access needle may become dislodged. In the case that such an event goes unnoticed, an arterial access needle can continue to draw blood from the patient while the dislodged venous access needle does not return blood to the patient.